1. Field of the Invention
The present invention relates to a machine and process for bending flat sheet metal into the form of a round tapered cylindrical tube, pole, or pipe that is commonly found around highways, parking lots and playing fields to support lights or signs.
2. Background
The use of metal poles for the structural support and placement of devices used by the utility and lighting industries has been a common practice for many years. Tapered metal poles in particular are widely implemented for the structural support of roadway illumination, vehicle parking lot illumination, and traffic signals. Metal poles are preferred over wooden poles for their structural properties, durability and aesthetic appeal. Other materials such as composites can be made to achieve similar structural properties as steel poles but require expensive adhesives to secure base plates and mounting fixtures to the parent pole.
Tapered metal poles are typically constructed in lengths ranging from 20 to 50 feet and typically have wall thicknesses that vary, according to length and loading conditions, from 0.12 inches to 0.25 inches. These poles typically have an apex diameter of 4 inches and a base diameter which is dictated by the length and taper. A preferred "standard tapered tube" is typically a hollow conical tube which typically has a 0.14 inch per foot diameter taper to a small diameter of about four inches, little to no surface defects, and one welded longitudinal seam. The tube should have the same diameter at the top (the small diameter) regardless of the length or of the bottom diameter.
The American Society for the Testing of Metals, ASTM, has developed a standard for tapered steel tubes implemented for structural use, referred to as A595. The specification delineates the material composition, dimensional tolerances and testing procedures required to meet the A595 standard.
Several methods can be used in the construction of tapered metal poles. Most processes currently use a complicated series of forming operations and often require additional sizing to meet the ASTM standard for dimensional tolerances. As can be expected, the purchase price of a tapered round metal pole is reflected by the complexity of the forming method used in its manufacture. Thus, incentive for the development of a new mechanism which can produce round tapered metal poles faster and more economically than current market sources can provide is of great interest to the lighting and utility companies.
There exists a variety of manufacturing processes available to form tapered tubes. One process which is commonly used to produce tapered tubes is rotary swaging. Rotary swaging is a process for reducing the cross-sectional area or otherwise changing the shape of bars, tubes, or wires by repeated radial blows with one or more pairs of opposed dies. Such swaging machines are generally large and heavy so that they can handle the large forces and vibrations associated with swaging. There are several problems which prevent swaging large diameter tubes from being practical. Not only is the machine required to swage large diameter tubes large and takes up a lot of space, it is difficult to produce lengthy large diameter tapered tubes of uniform wall thickness. Finally, to reduce the effects of work hardening and to prolong the service life of the swaging machine, rolls and backers used in the cold swaging process must be stress relieved periodically.
Another process used to produce tubes and pipe is roll forming. This process, however, cannot be easily modified to produce round tapered poles.
The following patents employ a variety of methods to produce round tapered tubes. None, however, utilize the Applicant's machine and process for forming round tapered pipe from flat sheet metal. U.S. Pat. No. 4,971,239, "Method and Apparatus for Making Welded Tapered Tubes," describes a process of taking a flat sheet of metal, bending it into a `U` shape, bending the top together and welding it to form a tapered oval. The tapered oval is then formed into a tapered round tube by a tapered outside die. Although this patent utilizes pre-forming of the sheet before it is worked into a tapered tube, this process requires the use of large, expensive presses to form the `U` and requires a series of fin rolls to deform the section into an oval shaped tapered tube.
U.S. Pat. No. 3,452,424, "Forming and Welding Tapered Tubes" uses the same ideas as the above patent. However, once the tapered oval is formed, this patent utilizes an inside mandrel which is forced into the inside of the tube to form the round tapered shape. This process also requires the use of large expensive equipment to form the `U` and to force the mandrel into the tube.
U.S. Pat. No. 4,846,392, "Continuously Variable Speed, Die-Drawing Device and Process for Metal, Composites, and the Like, and Compositions Therefrom" describes an elaborate machine for producing tapered poles from a continuous sheet of rolled sheet metal. This machine is an extruding type device and the patent makes no mention of pre-forming the sheet.
U.S. Patent No. 3,920,173, "Method and Machine for Manufacturing Shaped Parts From Flat Sheet Metal," and U.S. Pat. No. 3,802,235 "Machine and Method for Forming Tapered Tubes," both describe a process in which a mandrel is employed to shape a trapezoidally cut sheet of metal.
U.S. Pat. No. 4,622,841, "Method of Forming Long Metal Tubing to Tapered Shape" and U.S. Pat. No. 3,802,240, "Device for the Conical Tapering of Circular Cross-sectioned Elongated Work Pieces" are both patents for a method and device for producing tapered tubes. However, each of these devices requires tubing or pipe as a work piece rather than flat trapezoidal sheets of metal.